As examples of technology to realize high-speed and high-capacity optical transmission, a polarization multiplexing technology and a multi-value modulation technology are known. For example, the polarization multiplexing technology transmits signals assigned or mapped to two orthogonal polarized waves. Meanwhile, the multi-value modulation technology modulates light to be transmitted by using a modulation scheme available to transmit information of multiple bits in one symbol time. The modulation scheme may be a phase-shift keying including QPSK or a quadrature amplitude modulation of 2M-QAM (where, M represents the number of multi-levels and is an integer of two or more).
In addition, a digital coherent reception technology using digital signal processing may be applied to a reception side of an optical signal. In the digital coherent reception technology, a received signal light is mixed with a local oscillation light of a local oscillator source by 90 degrees optical hybrid. Thereby, information indicative of the amplitude and the phase of the optical signal with reference to the local oscillator light is extracted. The extracted information (signal) is digitalized by an analog-to-digital converter (ADC), and the digital signal is demodulated by using a digital signal processing. The digital signal processing is possible to compensate a waveform distortion (in other words, degradation of a signal quality) of received signal light due to wavelength dispersion, polarization mode dispersion, and the like of the optical transmission line.
In contrast, since a polarization state of a transmitted optical signal varies with time, it is difficult to compensate for degradation of the signal quality due to a polarization dependence loss (PDL), which occurs in an optical transmission line, an optical repeater, or the like, by using digital signal processing on the reception side. For this reason, the PDL is a major factor that limits the transmission capability of an optical signal.
In order to absorb the degradation of the signal quality due to the PDL, a scheme to average the polarization states of a transmitted optical signal (or a scheme corresponding to an averaging process) has been proposed (for example, see JP 2009-89194 A and JP 2010-109705 A).
JP 2009-89194 A discloses that data to be transmitted with two orthogonal polarizations are interleaved. Thus, according to JP 2009-89194 A, even if the PDL is present in an optical transmission line, an optical repeater, or the like, it is possible to average bit error rates (BERs) between the polarizations.
Meanwhile, JP 2010-109705 A discloses a high-speed polarization scrambling process achieved by a polarization scrambling process with digital signal processing. Thus, according to JP 2010-109705 A, similar to JP 2009-89194 A, even if the PDL is present in an optical transmission line, an optical repeater, or the like, it is also possible to average the BERs between the polarizations.
As examples of technology to monitor the PDL, technologies disclosed in JP 2009-133840 A and JP 2010-226499 A are known.
However, according to the technologies to average the polarization states of a transmitted optical signal by using a scrambling process, as disclosed in JP 2009-89194 A and JP 2010-109705 A, a maximum penalty due to the PDL of the transmitted optical signal may be absorbed but the averaging process merely achieves a small improvement in the BERs. Meanwhile, in order to further improve in the BER, the varying of the polarizations may be speeded-up on the transmission side in the technology disclosed in JP 2010-109705 A. However, the reception side may be unavailable to follow (or track) a change in the polarizations. Hence, the influence for the optical signal on the reception characteristic is large.
Both of JP 2009-133840 A and JP 2010-226499 A merely disclose technologies to measure (or monitor) the PDL of an optical transmission line.